1. Field of the Invention
The present invention relates to User Equipment (UE) and communications systems in which the UE registers with a network. More particularly, the present invention is concerned with the problem of UE registration with an Internet Protocol (IP) Multimedia Subsystem (IMS) network expiring during an IMS call or other session utilizing Circuit Switched (CS) bearers for the transport of the media, when re-registration is not possible.
2. Background to the Invention
The following abbreviations will be used throughout this disclosure:                AS Application Server        CS Circuit Switched        DTM Dual Transfer Mode        E-UTRAN Enhanced-Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) (aka Long Term Evolution (LTE))        GERAN Global System for Mobile communications (GSM) Enhanced Radio Access Network        ICS Internet Protocol (IP) Multimedia Subsystem (IMS) Centralized Services        IMS IP Multimedia Subsystem        MGW Media GateWay        MO Mobile Originated        MSC Mobile Switching Centre        MT Mobile Terminated        OMA DM Open Mobile Alliance Device Management        PS Packet Switched        SC Service Continuity        SCC Service Centralization and Continuity        S-CSCF Serving-Call Session Control Function        UE User Equipment        
Internet Protocol (IP) Multimedia Subsystem (IMS) networks are well known. They have an architecture, and use protocols, which deliver IP multimedia to mobile users (that is, to the user's mobile User Equipment (UE)). One part of the interaction between UE and an IMS network is that the UE registers with the network. More particularly, the UE registers a public user identity with the network, and this registration is for a registration period.
Currently, when an IMS Centralized Services (ICS)/Service Continuity (SC) UE (i.e. an IMS compatible UE) moves from one Access Network to another, there is the possibility that the UE's IMS registration will expire if the media is being carried via a Circuit Switched (CS) bearer, and the transferred-in Access Network does not support simultaneous Packet Switched (PS) and CS bearers, e.g. Global System for Mobile communications (GSM) Enhanced Radio Access Network GERAN not having Dual Transfer Mode (DTM). This is because a PS bearer is necessary to carry the IMS re-registration request. However, if the ongoing session (call) is utilizing a CS bearer, simultaneous use of a PS bearer is not possible.
This problem also occurs when the UE is in Idle Mode in a network that does not support simultaneous PS and CS and is IMS registered. In that case, subsequent call establishment using ICS I1 procedures will result in an ICS session that relies on the UE's IMS registration. If the registration subsequently expires, the remote leg established via the IMS will be released.
The conventional default expiration time requested by the UE for IMS registration (i.e. the default registration period when the UE registers with the IMS network) is specified in 3rd Generation Partnership Project (3GPP) Technical Specification 24.229 (3GPP TS 24.229) as 600,000 seconds. The entire contents of 3GPP TS24.229 is hereby incorporated by reference. This default period may be reduced by the Registrar in the network (Serving-Call Session Control Function (S-CSCF)) depending on operator policy in the 200 OK message that is sent in response to the REGISTER message from the UE. The UE may re-register at any time following the reception of the 200 OK message, but the default behavior described in 3GPP TS 24.229 is that the UE should re-register in IMS, 600 seconds before expiry of the registration if the expiry time (i.e. initial registration period) was greater than 1200 seconds, or when half the registration time (registration period) has expired if the registration period was less than 1200 seconds. Thus, 3GPP TS 24.229 specifies that re-registration should take place at a time determined by (i.e. dependent upon) the length of the initial registration period. However, this threshold for re-registration of 600 s or less before the expiry time may not be sufficient for a UE engaged in an ICS call, as it is possible that the call may exist in a non simultaneous PS and CS capable Access Network (when the UE cannot contact the IMS to re-register), for longer than 600 seconds.